1. Field of the Invention
The present invention relates to an AC motor drive apparatus connected to inverters.
2. Description of the Prior Arts
The motor devices having the circuits shown in FIG. 1(a) and (b) have been known.
In FIG. 1(a), the reference numeral (100) designates a stator having plural groups of first and second AC windings (101),(102) in three phases; (120) designates a rotor having DC excitation windings (121),(201),(202) respectively designate a plurality of inverter units connected to the AC windings; (31),(32) respectively designate smoothing reactors which are connected in series to the DC input circuits of the inverter units. The pairs of DC input terminals (p.sub.1, n.sub.1) (p.sub.2, n.sub.2) of the inverter units (201), (202) are respectively connected through the smoothing reactors (31),(32) in parallel to the DC terminals (P, N) of the common DC power source (300).
In FIG. 1(b), a plurality of the inverter units (201), (202) are connected in series to the DC input terminals (p.sub.1, n.sub.1) (p.sub.2, n.sub.2). The series parts are connected through the DC reactor (30) to the common DC power source (300).
The inverter units (201),(202) and the DC power source (300) can be an electric valve circuit (200) such as an electric valve bridge having electric valves (2a) to (2f) as shown in FIG. 4(a).
In FIG. 1(a) or (b), the frequency of the torque pulsation can be increased by increasing commutation rate per 1 cycle of the motor (1) (switching times of the electric valve per each cycle of total groups of the inverter units) and the phase difference (electric angle of 30 degree in three-phase two groups) is given between the first AC winding (101) and the second AC winding (102) in order to decrease the torque pulsation coefficient and the inverter units (201),(202) are conductively controlled with phase difference each other.
However, in the parallel connection of FIG. 1(a), the voltage pulsation synchronized to the commutation of the inverter units are generated in the voltage V.sub.pn.sbsb.1 between the DC terminal (p.sub.1, n.sub.1) of the first inverter unit (201) and the voltage V.sub.pn.sbsb.2 between the DC terminal (p.sub.2, n.sub.2) of the second inverter unit (202), and the voltage pulsation respectively have the phase difference. When the frequency of the AC windings is low for example, low revolution rate of a motor), the frequency of the pulsation is low whereby the pulsating transversal current is caused depending upon the instantaneous value difference of the inverter voltages V.sub.pn.sbsb.1 and V.sub.pn.sbsb.2. That is, the first invertor input current I.sub.1 and the second inverter input current I.sub.2 are changed to I.sub.1 &gt;I.sub.2 or I.sub.1 &lt;I.sub.2 depending upon the rotation.
When the mutual change of the currents are controlled by the reactors (32), (31), the smoothing reactors should be larger depending upon lower frequency. This is uneconomical.
When the pulsating voltage of the DC power source (300) is found, the beat phenomenon is caused by the power source input current pulsation caused by the pulsating voltage and the commutation of the inverters. That is, when the commutation is given at the peak value of the power source input current, the commutation overlapping angle of the inverters is increased. The DC voltage of the inverter unit is increased and the mean value of the DC input current of the inverter unit is decreased. Moreover, two inverter units are commutated in phase difference. When one inverter unit is synchronized to the peak value of the power source current pulsation, the other inverter unit is synchronized to the bottom value of the power source current pulsation. The phase relation for generation of beat is shifted. The beat is maximum when the commutating frequency of the inverter unit is substantially same with the pulsating frequency of the power source (300). The beat is caused at relatively high revolution rate of the motor. (relatively high frequency of AC winding frequency). The beat frequency is low frequency and it appears as the pulsating transversal current of the DC input currents. The smoothing reactor for preventing the beat should be large. This is also uneconomical.
The inverter pulsation in low frequency zone and the beat pulsation in high frequency zone appear as the torque pulsation of the motor. The purpose for decreasing the torque pulsation is not attained.
In the serial connection system of FIG. 1(b), two inverter units (201), (202) are connected in series whereby in DC voltage V.sub.pn.sbsb.1, and V.sub.pn.sbsb.2 of the inverters are serial sum and the resultant inverter voltage for the DC power source (300) is V.sub.pn.sbsb.1 + V.sub.pn.sbsb.2, so as to be considered as one inverter. The DC input currents of the inverters are equal whereby the transversal pulsation of DC current as found in the case of FIG. 1(a) is not caused.
However, the inverter units have potential difference .DELTA.V as DC current, whereby the mean potential differences .DELTA.V= (V.sub.NU.sbsb.2 - V.sub.NU.sbsb.1) is caused between the neutral potentials V.sub.NU.sbsb.1, and V.sub.NU.sbsb.2 of AC windings (ideal neutral potential in .DELTA. connection such as ring connection). (In FIG. 1(a) V.sub.NU.sbsb.1,= V.sub.NU.sbsb.2). Accordingly, it is necessary to give insulation having higher insulation withstand voltage for .DELTA. V as the earth insulation of AC windings (insulation to iron core) and the insulation between windings.
When the serial numbers (G) of inverter units are increased, higher degree of insulation withstand voltage is required. However, when the insulation is increased, the ratio of the insulator in the slots is increased to decrease the effective sectional area of the conductor, whereby the uneconomical motor is formed and the motor having high leakage inductance coefficient is formed and the commutating characteristics of the inverter units are adversely affected.
When the leakage inductance is increased, the control response velocity of the motor and the input current allowance change velocity dI/dt (di/dt of ripple; current rising dI/dt and current falling dI/dt) are decreased, whereby the characteristics of the motor are lowered. The deterioration of characteristics is especially trouble in the case of a quick response motor which sould have high response such as a motor for mill.
As described above, in the case of FIG. 1(a), the transversal pulsation is caused in the DC input current. This is uneconomical for preventing the transversal pulsation. The characteristic of torque pulsation is deteriorated.
In the case of FIG. 1(b), high insulation withstand voltage of the AC windings is required to be uneconomical. The response is also deteriorated.